Somebody who's not quite mentally competent
Similarly you may use the phrase, a few fries short of a happy meal
Weird Bus Dude: Hey wanna be my friend????? (waves spastically)
B: Can't Say I do
S: Well that guy is a few slices short of a loaf of bread!
by Wicked Witch of the West February 28, 2009
mugGet the A Few Slices Short of a Loaf of Breadmug.